In battery operated equipment, long battery life is very important. One factor influencing battery life is the efficiency of the voltage regulators used to power the different subsystems in the equipment. Switching regulators (SWR) provide among the highest efficiency, and they are widely used. However, switching regulators are relatively expensive and take-up a relatively large amount of printed circuit board area. As a result, linear low drop out regulators (LDOs) are widely used where lower efficiency can be tolerated. This results in systems that typically have a few SWRs and a large number of LDOs.
The LDOs operate with reasonably high efficiency if the difference between their input voltage, Vin, and output voltage, Vout, is small relative to Vout. This voltage difference is referred to as an overhead voltage Vovh, where Vovh=Vin−Vout. Neglecting ground current, the efficiency, Eff, may be expressed as followsEff=(Pout/Pin)˜(Vout/Vin)=Vout/(Vout+Vovh)=1/(1+Vovh/Vout).
If LDOs are used between the battery and the subsystems, the efficiency changes as the battery voltage decreases through the discharge cycle. The efficiency starts out low for a fully charged battery, and increases as the battery voltage, Vbat, decreases toward its minimum voltage.
Combining an SWR and LDO in a cascade fashion, where the SWR's input is the battery voltage and the SWR's output voltage provides the LDO's input voltage, is often used to improve the overall efficiency. An advantage of such a combination is that, as the LDO operates from the regulated output voltage of the SWR, the LDO's overhead is relatively constant and may be chosen low enough so as to provide a reasonably high overall efficiency for the combination. There also are advantages inherent to linear regulators, such as, for example, low noise, low ripple, and fast transient response. The output voltage of the SWR, Voswr, is typically set to Vout+Vovh, where Vovh is a fixed voltage. The SWR's control loop may either control the Voswr to equal Vout+Vovh, or regulate the Voswr so as to provide the required constant Vovh by forcing the difference of Voswr and Vout to be Vovh.
FIG. 1 shows one prior art solution, where the Voswr is controlled to be equal to Vref2=Voutmax+Vovh. This solution is sensitive to the Vout tolerances, and should be designed for Voutmax. The switching regulator is typically a buck regulator, but it may other types of regulators, such as a boost regulator or a buck-boost regulator, for example.
FIG. 2 shows the different voltages in a SWR-LDO regulator combination. It shows the different components of Vovh. The largest component is denoted as Vdsmin. This is the minimal drain-source voltage of PFET pass device 106 that ensures that device 106 operates in its saturated (pentode) region. (PFET stands for p-channel field-effect transistor.) This is important because operating the pass device in this region provides the LDO with high ripple rejection and good load transient response. Using a drain-source voltage larger than Vdsmin may be good for ripple rejection, but will result in a decrease in efficiency. If the drain-source voltage of the pass device 106 is lower than Vdsmin, the pass device operates in its triode mode with greatly reduced ripple rejection and degraded load transient response.
The value of Vdsmin depends on process parameters, temperature, output current and pass device size. In many prior art solutions Vovh, is chosen to be large enough to cover Vdsmin under worst case process parameters, junction temperature, highest output current, and smallest pass device size. Typically, Vdsmin may vary two to three times over the full parameter space. Designing Vdsmin for the worst case may result in a system that has lower than optimal efficiency for most cases and under most operating conditions.
The next, typically much smaller component of Vovh is the SWR's ripple voltage, denoted as Vswrripple/2 in FIG. 2. Switching regulators generate a ripple voltage at their output due to their switching nature. This is typically expressed as a peak-to-peak (pp) ripple voltage around their DC output voltage. Half of this pp ripple voltage, Vswrripple/2, is part of Vovh. Vswrripple depends on the SWR's external storage components, such as capacitor C 108 and an inductor (not shown), and also depends upon the switching frequency. The next component of Vovh is the maximum load transient excursion of Voswr during the maximum specified positive load transient, Voswtrmax. Voswtrmax depends on the inductor (not shown) of the SWR, the value and ESR (equivalent series resistance) of the capacitor C 108, and the speed of its control loop. The final component, dVoswr, is the maximum deviation of Voswr from its nominal value due to component tolerances, temperature variation, and static line and load regulation. If the Voswr is set by this Vovh above the Voutmax, the LDO's pass device should operate in its saturated (pentode) region under all operating and ambient conditions with high efficiency, high ripple rejection and good load transient response.
FIG. 3 shows an alternative prior art implementation. In this implementation, the control loop of the SWR regulates the Voswr to be Vovh above the actual Vout of the LDO. This is accomplished by Error amplifier 102 forcing Voswr=Vout+Vovh. An advantage of this solution, compared to the one shown in FIG. 1., is that Voswr tacks the actual Vout with the constant Vovh overhead. As the tolerance of Vout is not part of the Voswr control loop, Voswr and Vout are closer to each other, resulting in higher efficiency. However, a disadvantage of this solution is that the overhead, Vovh, is constant and should be designed to meet worst case conditions over process, temperature, tolerances, and LDO operating conditions. This often results in over-design and lower efficiency for most operating conditions.